Any patient over the age of 18, admitted to the ICU and receiving antibiotics for an assumed infection can be enrolled into this trial. Informed consent has to be obtained in writing from the patient of his/her relatives prior to inclusion.

When procalcitonin has reached a peaklevel of above 1,0 ng/ml and the value has decreased to below 0,25 ng/ml, there will be adviced to stop antibiotics (absolute decrease).
A second intervention rule is with a decrease of 90% of the peak value there will also be advised to stop antibiotics (relative decrease).
When the procalcitonin peaklevel is below 1.0 ng/ml, a decrease to below 0.1 ng/ml is needed, before there will be advised to stop antibiotics (absolute decrease).

- Primary outcome

1. Consumption of antibiotics expressed as the Defined Daily Dosage and duration of antibiotic therapy expressed in days of therapy. In case of multiple antibiotic therapies the therapy that will be used the longest will be scored in duration of therapy;
2. 28-day mortality.

- Secondary outcome

1. The percentage of new episodes of bacterial infections for which antibiotics are prescribed per 1.000 ICU days;
2. Length of ICU stay, measured after discharge from the ICU in days;
3. Acquisition costs of antibiotics (in Euro’s);
4. Costs of procalcitonin (in Euro’s).

- Timepoints

An interim analysis will be performed after enrolment of the first 750 patients.

In all patients antibiotics will be started based on a clinical suspicion of infection or microbiological evidence of an organism. This decision is fully at the discretion of the team and should be made in the same way during the trial as under. Once antibiotics are administered for suspected or proven bacterial infection, a serum sample (T0) will be obtained and patients or their next of kin be asked for informed consent. If informed consent is obtained, the patient will be randomized to the standard therapy arm (control group) or the procalcitonin (intervention) arm. Randomization will be stratified for diagnostic group and centre. If a patient is randomized for the control group, no procalcitonin measurements will be performed. If a patient is randomized for the procalcitonin group, procalcitonin will be measured in the T0-serum sample. On the following days the treating physician will be given daily procalcitonin values until ICU-discharge or until the third day after al systemic antibiotics have been discontinued. Along with daily procalcitonin values the physician will also receive a printed, non-binding advice to consider stopping the prescribed antibiotics if procalcitonin has decreased to 90 % of its peak value measured during this episode or has dropped bellowed the defined values specified in the stopping rules. As for the control group, the physician will receive daily laboratory values as requested and no additional advice.